In a rolling step, a strip meanders by moving outward in a width direction of a rolling mill in some cases. Generally, in a hot rolling line, multiple rolling mills are arranged in tandem and the strip is held by the rolling mills during a so-called steady rolling, that is a period from when a leading end of the strip being rolled passes the rolling mill at the last stage until the tail end of the strip enters the rolling mill at the first stage. Accordingly, significant meandering of the strip rarely occurs.
However, after the tail end of the strip passes through each of the rolling mills, the meandering of the strip suddenly begins due to a loss of the holding force applied by the rolling mill from which the strip has just left. As a result, the strip has tail pinching in which the tail end is rolled while being folded down due to reasons such as contact with a side guide provided on an entry side of the next rolling mill. Such tail pinching damages a work roll. If the rolling is continued in this state, the damage on the work roll is transferred onto the strip and the quality of the strip deteriorates. Accordingly, the work for replacement of the work roll is required. This leads to reduction in productivity and yield of the strip.
A technique of controlling the meandering of the strip during the rolling is an important technique not only from the viewpoint of preventing rolling failures such as the tail pinching described above but also from the viewpoint of stable rolling which leads to improvement in productivity and reduction in manufacturing cost. Therefore, rolling methods for controlling the meandering of the strip to prevent the meandering from causing the tail pinching have been heretofore provided, and such rolling methods are disclosed in Patent Documents 1 to 4 for example.
In Patent Document 1, a skew angle of a conveyed strip with respect to the center line of a rolling mill is detected and thereafter meandering control of the strip is performed by adjusting screw-down leveling on the basis of the detected skew angle.
Moreover, Patent Document 2 uses a tensile force measuring roll capable of coming into contact with a strip, and makes measurements of vertical forces acting on left and right ends of the tensile force measuring roll, a thrust force acting in a roll axis direction of the tensile force measuring roll, and a threading position of the strip in a strip width direction on the tensile force measuring roll. Then, a left-right tensile force difference of the strip is calculated based on the vertical forces, the thrust force, and the threading position of the strip in the strip width direction. Thereafter, meandering control of the strip is performed by adjusting screw-down leveling on the basis of the calculated left-right tensile force difference of the strip.
Furthermore, in each of Patent Documents 3 and 4, a meandering amount of a strip is calculated based on the positions of left and right strip end portions of the strip which are detected by using multiple split rolls and thereafter meandering control of the strip is performed by adjusting a roll bender amount and screw-down leveling on the basis of the calculated meandering amount of the strip.